1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and the like, utilizing an electrophotographic or electrostatic recording system.
2. Description of the Related Art
In conventional image forming apparatuses, such as copying machines, printers and the like, a latent image formed on an image bearing member comprised of an electrophotographic photosensitive body or an electrostatic recording dielectric body is visualized by using toner as developer, which is powder. Recently, in order to cope with compactness, simplification, energy reduction and environmental problems, an image forming apparatus in which a cleaning container or a waste toner container is omitted or in which generation of ozone due to discharging is reduced has been noticed.
Now, as an example, a cleanerless image forming apparatus of a reversal developing system with nonmagnetic negatively electrified toner using noncontact developing and noncontact electrifying type will be described with reference to FIG. 6.
FIG. 6 is a sectional view showing a schematic construction of a conventional image forming apparatus using dry type one-component developing apparatus.
As shown in FIG. 6, such an image forming apparatus comprises a photosensitive drum 101 as a latent image bearing member rotated in a direction shown by the arrow p, an electrifier 102, an exposing device 103 for supplying image information, a developing device 104, a transferring device 105 as transferring means and a fixing device 106.
The developing device 104 includes a developing roller 110 as a developer carrying member rotated in a direction shown by the arrow q to carry toner as developer to an opposed area between the photosensitive drum 101 and the developing roller. The developing roller 110 is a rigid body having conductivity, and a gap between the developing roller and the photosensitive drum 101 is selected to be about 300 xcexcm. Further, around the developing roller 110, there are provided a peeling roller 111 rotated in a direction shown by the arrow r and serving to supply nonmagnetic one-component toner onto the developing roller 110 and to peel the toner from the developing roller 110, a regulating blade 112 for applying desired electrifying amount to the toner on the developing roller 110 and for regulating a toner amount, and a developing bias voltage power source 113 for applying developing bias obtained by overlapping an AC voltage and a DC voltage to the developing roller 110. In this example, a DC voltage of xe2x88x92350 V and rectangular waveform AC voltage having a frequency of 1800 Hz and peak-to-peak voltage of 1600 V are applied to the developing roller 110 from the developing bias voltage power source 113. Further, the peeling roller 111 is constituted by forming a foam sponge around a metallic core support shaft.
The electrifier 102 includes an electrifying roller 120 as electrifying means rotated in a direction shown by the arrow s and having a conductive metal core (not shown) and a conductive elastic layer (not shown) covering the metal core, and an electrifying bias voltage power source 121 for applying a DC developing bias voltage to the electrifying roller 120. In this example, a DC voltage of xe2x88x92600 V is applied to the electrifying roller 120 from the electrifying bias voltage power source 121. The electrifying roller 120 is contacted with the photosensitive drum 101 and is rotated with peripheral speed difference with respect to the photosensitive drum. In this example, by using the photosensitive drum having a charge injecting layer on the surface thereof and the electrifying roller having low resistance, the photosensitive drum can be electrified with potential substantially the same as the electrifying bias voltage without discharging.
As shown in FIG. 6, the photosensitive drum 101 is rotated in the direction p and is uniformly charged to xe2x88x92600 V on the surface thereof by the electrifying roller 120, and, thereafter, an electrostatic latent image is formed on the photosensitive drum 101 by the exposing device 103. The electrostatic latent image is visualized as a toner image by the toner carried by the developing roller 110 opposed to and contacted with the photosensitive drum 101 from the developing device 104. Thereafter, the toner image on the photosensitive drum 101 is transferred onto a transfer material 107 as a recording medium such as paper or OHP sheet by the transferring device 105. Then, by the fixing device 106, the toner image is dissolved and ultimately fixed onto the transfer material 107.
On the other hand, after the transferring, residual toner (referred to as xe2x80x9ctransfer-residual tonerxe2x80x9d hereinafter) remaining on the photosensitive drum 101 is often electrified with weak positive polarity because it is subjected to the transferring bias voltage. When the transfer-residual toner reaches an abut area (referred to as xe2x80x9celectrostatic charge nipxe2x80x9d or xe2x80x9celectrifying nipxe2x80x9d hereinafter) between the electrifying roller 120 and the photosensitive drum 101, the transfer-residual toner is returned to negative polarity while being subjected to friction and agitating and is carried to a developing station by the rotation of the photosensitive drum 101. The toner carried to the developing station is collected on the developing roller simultaneously with the developing and is used again for visualizing a new electrostatic latent image.
However, in the above-mentioned conventional image forming apparatus, there may arise the following problem during image formation.
In such an image forming apparatus, at the developing station, an electric field for biasing the toner from the photosensitive drum to the developing roller with respect to an image portion and a nonimage portion on the photosensitive drum and an electric field for biasing the toner from the developing roller to the photosensitive drum with respect to the image portion and the nonimage portion on the photosensitive drum are in generated alternately. When the electric field for biasing the toner from the developing roller to the photosensitive drum is being applied to the developing roller, after the electrostatic latent image on the photosensitive drum passes the developing area, the toner will also be adhered to the nonimage portion. Consequently, during the transferring, a portion of the transfer material corresponding to the nonimage portion is contaminated by the toner, with the result that a high quality image output cannot be attained.
Further, when the negatively electrified toner passed through the electrifying nip is collected at the developing station, since the electric field for biasing the toner from the developing roller to the photosensitive drum is generated, the toner cannot be collected completely.
To solve the above problem, it is considered that a developing method is improved as a contact developing system in which an elastic developing roller to which only DC voltage is applied is contacted with the photosensitive drum. In this case, DC voltage of xe2x88x92350 V is applied to the developing roller.
As a result, since the electric field for biasing the toner from the developing roller to the photosensitive drum is always generated with respect to the image portion on the photosensitive drum and the electric field for biasing the toner from the photosensitive drum to the developing roller is always generated with respect to the nonimage portion on the photosensitive drum, the toner is not adhered to the nonimage portion, thereby achieving a high quality image output.
Further, when the toner passed through the electrifying nip is collected at the developing station, since the electric field for biasing the toner from the photosensitive drum to the developing roller is always generated, the toner can be collected completely.
However, at a contact area (referred to as xe2x80x9cdeveloping nipxe2x80x9d hereinafter) between the developing roller and the photosensitive drum, the latent image formed by the exposing portion may be distorted. Such a phenomenon occurs due to the fact that the electrifying process is effected not only by the electrifying roller but also by the developing roller contacted with the photosensitive drum.
An object of the present invention is to provide an image forming apparatus in which an image bearing member is prevented from being electrified by a developer carrying member.
Another object of the present invention is to provide an image forming apparatus in which an electrostatic image on an image bearing member is prevented from being distorted by a developer carrying member.
A further object of the present invention is to provide an image forming apparatus in which an image bearing member is electrified effectively by an electrifying member.
A still further object of the present invention is to provide an image forming apparatus in which a developer carrying member can perform a developing operation and a cleaning operation simultaneously.